The present invention relates to a method for forming a coating film at uniform thickness on metallic articles having projections and depressions defined by or defining the projection, particularly gears and spline shafts. The present invention also relates to a method of manufacturing coated articles such as coated gears and spline shaft, and to a coating apparatus.
Metallic spline shafts and metallic gears for use in motive power or motion transmission are formed with a resin coating to impart thereto wear resistance, cushioning property, soundproof property or muffling property. In the case where such a resin coating is formed by fluidization dipping process, the resulting coating become large in thickness because of large thermal capacity of the articles (such as gears etc.), while an attempt to make the coating into a thin film would cause the coating levelness to lower, making it difficult to form a uniform coating with high precision. Therefore, for example, after coating to a film thickness of 300 μm or more (in particular, about 500–600 μm) by fluidization dipping process, the coating is subjected to machining process to form a coating film of about 200±20 μm. However, this method involves the machining process at the crests and troughs, or roots, of teeth of gears, resulting in not only lowered production efficiency but also increased cost.
Meanwhile, it is also practiced that a powder bed is formed by applying an electrostatic powder coating to gears or spline shafts, and then a heating process is performed to form a coating film. With this method, unfortunately, larger-sized gears or spline shafts would take prolonged time for elevation of the object temperature by an oven. Moreover, since the temperature elevation begins with the surface of the powder bed in the oven, rapid heating, if applied, would involve scaling and/or flaking, namely detachment, of the coating film and/or occurrence of pinholes, thus making it difficult to form a large-thickness coating film uniformly.
In order to solve the problems with the oven, electrostatic powder coating process and high frequency induction heating process are practiced in combination to form a coating film. For example, a Japanese publication JP-A-10-296182 discloses to form a resin powder bed (thermosetting resin powder bed of epoxy resin or the like) by electrostatic powder coating process on a continuous metallic body of a metal wire, metal tube, metal bar and the like, and then fuse the inside layer of the powder bed by induction heating process to have the inside layer adhere to the metallic body while forming a semi-fused resin layer at the top surface of the powder bed, and perform further heating to finally form a completely fused resin coating film. Japanese Patent No. 2725168 discloses a method for manufacturing an anticorrosion-coated steel stranded wire including the steps of temporarily undoing a steel stranded wire into a core wire and individual side wires, depositing resin powder onto the core wire and the side wires, heating the core wire and the side wires during the process of re-stranding them so that an incompletely set coating layer is formed around the outer periphery and between the core wire and each side wires, and further heating the re-stranded steel wire so that the coating layer is completely set.
Further, the Japanese publication JP-B2-56-34153 discloses an insulating-coating formation method including the steps of depositing a powder coating material onto an article having an uneven surface, while the article is being rotated, within an electrostatic fluidized dipping vat in which a 100–250 mesh powder coating material is fluidized and electrified, removing unnecessary powder coating material from projections of the article by a brush-equipped suction means while rotating the article, and heating the article by induction heating of about 100–200 kHz to thereby fuse and fix the powder coating material. In this method, in order to ensure electrical insulation at depressions as in the motor rotor while avoiding the formation of coating film at projections, a thicker powder bed is formed by electrostatic-fluidized dipping process using a 100–250 mesh powder coating material (epoxy resin coating material), and then the powder bed of projections where the coating film is unnecessary is removed, leaving the thick film only at the depressions.
However, the electrostatic powder coating process and the electrostatic-fluidized dipping process, which use electrified powder particles, inevitably involve increased film thicknesses at crests (tip portions) of the teeth and reduced film thicknesses at troughs and side walls due to the electrical repulsion of the charged particles. In particular, removal of the powder bed at the projections by a brush-equipped suction means during the rotation of the article may cause, in some cases, the powder at the projections to fall into the depressed portions. As a result, even with induction heating, articles having a region having projections and depressions (e.g., toothed portions of gears and spline shafts, etc.) cannot be coated with a uniform coating film.